/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
  public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
      if(root == null) return null;
      if(root == p || root == q) return root;
      TreeNode left = lowestCommonAncestor(root.left,p,q);
      TreeNode right = lowestCommonAncestor(root.right,p,q);
      if(left != null && right != null) return root;
      return left == null ? right : left;
  }
}